Cleanup: move tag functions into own translation unit

Make dive.h a bit slimmer. It's only a drop in the bucket - but at
least when modifying tag functions not the *whole* application is
rebuilt anymore.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/command_edit.cpp

@@ -5,6 +5,7 @@
 #include "core/divelist.h"
 #include "core/qthelper.h" // for copy_qstring
 #include "core/subsurface-string.h"
+#include "core/tag.h"
 #include "desktop-widgets/mapwidget.h" // TODO: Replace desktop-dependency by signal
 
 namespace Command {

desktop-widgets/divelogexportdialog.cpp

@@ -17,6 +17,7 @@
 #include "profile-widget/profilewidget2.h"
 #include "core/save-profiledata.h"
 #include "core/divesite.h"
+#include "core/tag.h"
 
 // Retrieves the current unit settings defined in the Subsurface preferences.
 #define GET_UNIT(name, field, f, t)           \

desktop-widgets/simplewidgets.cpp

@@ -22,6 +22,7 @@
 #include "profile-widget/profilewidget2.h"
 #include "desktop-widgets/command.h"
 #include "core/metadata.h"
+#include "core/tag.h"
 
 double MinMaxAvgWidget::average() const
 {